Systems of this kind are known, for example from the U.S. Pat. Nos. 3,631,422, 3,747,086, 3,754,226, 3,820,104, 3,820,103, 3,790,945, as well as from the German Offenlegungsschrift No. 2,160,041. Since the surveillance system described in the German specification No. 2,160,041 substantially represents a combination of the systems described in the said U.S. patent specifications, only the Offenlegungsschrift will be dealt with in detail in the following. In the system described therein, the procedure applied in the simplest case is that at least one magnetic field is generated in the area of an exit door of a store, warehouse or the like. The objects sold in the store carry marking elements which may be affected by the magnetic field. A non-linear behaviour of the marking element may be produced in the commonest case, so that frequencies differing from the excitation frequencies or frequency and detectable by sensors are engendered if a proper sale has not been made.
In particular, the procedure applied in such case is that two alternating magnetic fields are engendered by appropriately aligned coils within the area of the exit door. The coils generating the magnetic fields are carefully tuned with respect to each other in such a manner that no mutual induction occurs, so that the fields cannot affect each other. If the marking element, which consists of magnetic material of high permeability, is then however brought between the oscillation components of the overall field formed within the exit area, said element normally has a direction enabling the two magnetic fields generated to drive the magnetic material of the marking element into the saturated state, that is to say at either side of the known hysteresis loop, since these are alternating magnetic fields. Since these magnetic actions have a considerable non-linearity, addition and subtraction signals are produced from the two excitation frequencies are transmitted in the form of electromagnetic radiation by the marking element. In a specified embodiment, the excitation frequency of one field amounts to 21 kc/s, and the excitation frequency of the other magnetic field amounting to 24.5 kc/s. It is possible to detect a corresponding differential frequency of 3.5 kc/s, which is transmitted by the marking element among many other frequencies, to appropriate sensors in resonance with this frequency, to produce a corresponding signal. What is essential in a system of this kind is merely that the two excitation frequencies do not affect each other mutually from the beginning and that no corresponding summation or differentiation signals are engendered, so that any mutual induction should be prevented painstakingly in the case of coils engendering the alternating magnetic fields. Upon application of one coil only, it is also possible to interpose appropriate filters between the coil and the excitation systems in question, so that non-linearities and the forming of corresponding modulation frequencies are prevented. On the other hand a system of this nature also operates if a single magnetic field is merely generated and brought into action on the marking element, because the marking element generates harmonics of the basic oscillation, which would not be present in the absence of the marking element, which may however be picked up and exploited for providing an appropriate warning.
It is already known moreover to affect the marking element upon proper completion of a sale, in such manner that the generation of harmonics -- or of the said summation and differentiation signals if the operation is performed with two different frequencies -- is prevented. For example, the marking element may consist of a magnetic material of high permeability which is brought together with a second ferromagnetic element of high coercivity. If a permanent magnetization is then imparted to this second element, for example in the area of the store cash register, this second magnetic element is able to keep the first magnetic element in a constant state of saturation, so that the action of the alternating magnetic fields in the area of the door can no longer have any effect, since the hysteresis loop is no longer passed through, so that the non-linearities are also suppressed.
It is evident that a system of this kind may operate properly only if it is assured that the marking element is always able to respond to the magnetic field or fields within the surveillance area, i.e. the magnetic fields should be so directed spatially that there is substantially no position for the marking element which can prevent the generation of the harmonics attributable to the non-linearities. It may well be assumed that the occasional user has no knowledge of the orientation of the magnetic fields in the surveillance area, i.e. normally in the area of the exit door. If, however, the object comprising the marking element is held so that one of the directional magnetic fields cannot act on the magnetic material of high permeability of the marking element, the forming of the summation or differential frequencies does not occur either and the transmission of a signal does not take place. In other terms, this means that the possibility of the presence of so-called blind spots cannot be excluded.